Fiber-reinforced composites manufactured by reinforcing plastic materials (called matrix resins) with various synthetic fibers are used widely in automotive application, aviation and space application, sporting and leisure goods application, and general industrial use. Fibers employed for the composites include inorganic fibers, such as carbon fiber, glass fiber and ceramic fiber; and organic fibers, such as aramid fiber, polyamide fiber and polyethylene fiber. These synthetic fibers are usually produced into filament, and later processed into various forms of reinforcement textiles, including a sheeted intermediate material called unidirectional prepreg which is manufactured by applying hot melt resin to fabric and winding it onto a drum, textile material manufactured by filament winding, woven fabric or chopped fiber.
Prospective fiber-reinforced composites are those manufactured of thermoplastic resins including polyolefin resins, nylon resins, polycarbonate resins, polyacetal resins, ABS resins, polyphenylene sulfide resins and polyetherimide resins among the matrix resins mentioned above, which attract the attention because of their good moldability and advantage in recycling. Of these resins, polyolefin resins have better moldability and chemical resistance and are more advantageous in manufacturing cost than other resins. Thus fiber-reinforced composites manufactured of the polyolefin matrix resins attract much attention and are expected to be applicable to various uses as a versatile material.
Reinforcement fibers are often used in a form of chopped fiber cut into 1 to 15 mm long. The chopped fiber should have sufficient cohesion when it is knead with a thermoplastic resin to be manufactured into pellets, and chopped fiber having insufficient cohesion cannot be fed constantly to pellet manufacturing. In addition, reinforcement fiber strands having insufficient fiber cohesion sometimes break to deteriorate the property of resultant fiber-reinforced composite. For preventing such troubles, number of techniques for imparting optimum cohesion to reinforcement fiber by applying sizing agents containing modified polyolefin resins as a base component have been proposed (see JP A 2006-233346 and JPA 6-107442) and widely employed in industrial fields.
On the other hand, reinforcement fibers are increasingly processed into a form called long-fiber pellet or into unidirectional sheet, tape or fabric to be impregnated with thermoplastic resins and molded in subsequent processes like as composite materials of thermosetting resins, in order to effectively achieve the properties of reinforcement fibers including tensile strength. In such cases, hot-melt thermoplastic resins should quickly penetrate into fiber strands, specifically, fill space between single fibers in molding fiber-reinforced composites in order to shorten the molding time and improve the physical properties of resultant composites.
However, sizing agents disclosed in the prior art have further exhibited deteriorated poor wetting and bonding characteristics between sized fiber and matrix resin in molding process to further deteriorate mechanical properties of resultant composites in some cases where polyolefin resins inherently having poor wetting propensity are employed as a matrix resin.
Under such situation, a sizing agent which improves the affinity between sized fiber and matrix resin to firmly bond the fiber and resin has been demanded in the field of fiber-reinforced composites containing thermoplastic matrix resins, especially polyolefin matrix resins.